The present invention relates to a method for suppressing resonance phenomena in the a-c network on the inverter side of a high-voltage d-c transmission system (HVDCTS). The invention further relates to an apparatus for carrying out the method.
With reference to FIG. 1, an HVDCTS is shown which is arranged between two a-c networks NA, NB and in which a first converter 1A of the HVDCTS which is connected in a first station (STA) to the first a-c network NA, is clocked by the network and is operated as a rectifier, impresses a predetermined d-c current i.sub.dA from the first a-c network and in a second station (STB), a second converter 1B which feeds into the second a-c network NB, is clocked by the network and is operated as an inverter, determines the level U.sub.dB of the transmitted d-c voltage.
In the preferred embodiment shown of a short coupling, the two stations are physically close together and the HVDCTS comprises only the d-c lines 2 and 3, a converter choke LA and LB being arranged at the d-c terminal of each converter. Each station contains a regulator 4A, 4B, the output signal of which determines, as the control voltage .DELTA.x.sub.A, .DELTA.x.sub.B, respectively, the control angle for the control unit STA and STB of the respective converter. The control units can be supplemented in the case of a digital control unit by supplementary devices, not shown, for instance, for linearizing the characteristic and/or for converting the given analog control voltage into a digital signal in the case of a digital control unit.
The control voltage .DELTA.x.sub.A determines the d-c voltage U.sub.dA at the d-c terminal of the converter 1A and therefore, if the d-c current i.sub.dA is set into the regulator 4A (current regulator), the control deviation of the d-c current fed-in by the converter 1A, the transmitted HVDCTS d-c current i.sub.d which is equal to the d-c current i.sub.dB which is taken from the converter 1B of the HVDCTS and is fed into the network NB, since in the case of a short coupling, power losses occurring in thyristors and chokes can be ignored (i.sub.d =i.sub.dA =i.sub.dB). To the regulator 4B of station B (for instance, a voltage regulator or quenching angle regulator) can be fed, in the interest of holding the voltage of the network NB constant, the control deviation of the voltage amplitude of this network or, in the interest of an efficiency as high as possible for the power transmission and low reactive power, the control deviation of the quenching angle of the converter 1B can be pre-set by a predetermined designed quenching angle value which is a maximum as far as possible (i.e., is close to the inverter out-of-step limit). Its control voltage .DELTA.x.sub.B determines the d-c voltage U.sub.db associated with the impressed HVDCTS current i.sub.d at the d-c terminals of the converter 1B and thereby, the voltage level U.sub.d of the transmitted d-c voltage which is taken off between the two converter chokes LA and LB and, in the case of a long distance HVDCTS line, preferably at the end of the choke LA facing away from the converter 1A.
The two stations are coupled to each other via the HVDCTS in such a manner that any change of .DELTA.x.sub.B and U.sub.dB in station B causes, in accordance with the voltage drop at the HVDCTS, a change of U.sub.dA and thereby, a current change in station A which must be equalized by the current regulator 4A. On the other hand, any change of .DELTA.x.sub.A or U.sub.dA in the station A brings about a change of i.sub.d and U.sub.dB in station B, to which the regulator 4B responds in station B. In order to keep interfering effects of this coupling as low as possible, it has already been proposed to impress on the output signal .DELTA.x.sub.A of the current regulator 4A a pilot voltage U.sub.dAv, which is taken off directly at the measuring stage for U.sub.d or is formed by means of a computing circuit as the model variable. Likewise, a pilot voltage U.sub.dBv can be impressed on the control voltage .DELTA.x.sub.B, which is formed in a different manner, for instance, by calculating the inductive d-c voltage drop of the converter 1B.
For the network NB, the HVDCTS operates in practice so that the voltage oscillations are reflected at the converter, wherein a small ripple of U.sub.d and i.sub.d is produced which is equalized by the current regulator 4A if this current regulator works fast enough. In FIG. 1, it is indicated that the network NB can contain a disturbing voltage oscillation which in this manner acts like an oscillation coupled into the HVDCTS circuit via an interference quantity U.sub.B for the control angle .DELTA.x.sub.B with a generally changed amplitude, phase and frequency.
If in the network, this interfering oscillation is located in the vicinity of the resonance frequency of the network and if the current regulator 4A is not able to equalize the resonance frequency generated in the d-c circuit, this interfering oscillation can thereby be fanned-up in such a manner that considerable ripple of U.sub.d and i.sub.d in the HVDCTS circuit comes about, in which the d-c voltage corresponding to the nominal voltage provided is superimposed by a "resonance oscillation" which can exceed, for instance, 30% of the nominal voltage and can ultimately lead to the situation that the entire system must be shut down. This is the case, for instance, in a specific case, in which the resonance frequency of the network NB is in the vicinity of the second harmonic of the network NB, it having been found that the interfering resonance frequency of the d-c circuit is approximately equal to the fundamental (60 Hz) of the 3-phase network NB.